Shape-Controlled Synthesis Of Ni Nanocrystals And Their Oxygen Evolution Reaction Catalytic Activity

Non-noble metal nanomaterials are widely used in industrial catalysts,biological separation,sensing due to their excellent catalytic,magnetic and optoelectronic properties.Since the size of the nanoparticles is small,their catalytic properties are completely different from the corresponding bulk materials.Thus,regulating the size of the nanomaterials is a means to optimize their catalytic properties.In fact,the properties of non-noble metal nanomaterials are also related to the morphology of nanocrystals?NCs?.The atomic coordination number,atomic bulk density and atomic arrangement with special morphologies are different,which makes the reactants adsorption and the activated product desorption on the surface of NCs catalyst different.Therefore,morphology regulation is also an effective means to optimize the performance of non-noble metal NCs catalysts.However,the regulation of morphology is fine,influenced by many factors,and has greater challenges than the regulation of size.There are still uncertainties in determining the influencing factors with controllable synthesis of non-noble metal nanomaterials,establishing the relationship between structure and performance,and understanding their growth mechanism,which require further research.In recent years,researchers have synthesized noble metal nanomaterials?such as Pd,Pt,Ag,Au,etc.?with various morphologies.However,the controllable synthesis of non-noble metal nanomaterials?such as Fe,Co,Ni,Cu,etc.?is very limited because the reduction potential of them is relatively negative result in requiring harsh reduction conditions.Moreover,the non-noble metal is active and oxidized easily.They may be oxidized simultaneously in the reduced process,which makes the nucleation overlap with growth,and obtains uneven nanoparticles.As a non-noble metal,Ni is abundant in the earth and is a highly efficient industrial catalyst.In addition,reduction potential of Ni is relative high,which makes it easy to be reduced.Based on this,this paper synthesized Ni NCs with different morphologies by wet-chemistry strategy.The OER performance of Ni NCs with different morphologies were studied and the relationship between morphology and property was explored.The transformation mechanism of Ni NCs with different morphology was explored.The main research contents and conclusions of this paper are as follows:?1?Ni NCs with different morphologies?nanocubes,cuboctahedra and octahedra?were prepared by wet-chemistry strategy with nickel acetylacetonate[Ni?acac?₂]as precursor,formaldehyde as reductant and capping agent,cetyltrimethylammonium bromide?CTAB?as capping agent,oil acid,oleylamine and dibenzyl ether as solvents.The XRD results show that the prepared Ni NCs are pure phase.SEM,TEM and HRTM results show that Ni nanocubes are covered by{100}facets,Ni cuboctahedra are covered by{100}and{111}facets,and Ni octahedra are covered by{111}facets.During the preparation process,we found that?1?formaldehyde was a strong reducing agent,and the by-product CO could selectively adsorbed on the Ni{100}facets as a capping agent.?2?The Ni NCs could transformed from 0D nanocubes to 2D nanorod with the amount of CTAB increasing.?3?Since the affinity of Cl^-for Ni{111}facets is weaker than that of Br^-,the Ni nanocubes could not be prepared with CTAC as the capping agent.?2?The eletrocatalytic performance of Ni polyhedra catalysts was investigated with using OER as a probe reaction.The OER performance test was characterized by cyclic voltammetry?CV?with a potential range of 1.23–1.73 V and a scan rate of 5 mV S^-1.Electrochemical measurements were conducted in 1 M KOH electrolyte without internal resistance?IR?compensation.CV curves showed that the overpotentials of Ni nanocubes,cuboctahedra,octahedra and spherical NCs are 382 mV,402 mV,412 mV,and 423 mV,respectively,at a current density of 10 mA cm^-2.The morphology of Ni NCs catalysts was stable without obvious change by SEM.?3?We explored the transformation mechanism of Ni polyhedron NCs and the key factors affecting the morphology.It was found that the effects of formaldehyde,temperature and time on the morphology of Ni NCs are significant and belong to thermodynamic control.?1?Since the CO generated through formaldehyde thermal decomposition can selectively adsorbed on the Ni{100}facets,the adsorption density of CO increases with increasing formaldehyde.This would result in the morphology of Ni NCs changed from cuboctahedra to naocubes or octahedra to cuboctahedra,that is,the proportion of{100}facets increases.This result further confirmes that CO could reduce the surface free energy of{100}facets,thus slowing down the growth rate of{100}facrts.?2?The elevated temperature weakened the binding force of CO on Ni{100}facets and desorbed CO from the surfaces of NCs.As a result,the adsorption density of CO decreases and the morphology of Ni nanocubes switched to cuboctahedra and octahedra.?3?Over time,the adsorption density of CO decreases and the thermodynamically unstable atoms on the surface of Ni NCs?such as the atoms with low coordination number on the corner site?migrate to the edges or surfaces with more thermodynamically stable with size increasing,so the morphology changed from nanocube to cuboctahedra.